One alternative to manual segmentation of medical images is atlas-based segmentation, in which the segmentation of a target image is derived from already segmented atlas images. The concept of atlas-based segmentation is based on the assumption that the spatial relation between two images closely resembles the spatial relation between the respective segmentations. Therefore a deformation that spatially aligns an atlas image with the target image can be used to propagate the segmentation of the atlas image. This propagated segmentation can then be considered as an estimate of the unknown true segmentation of the target image. In many applications, multiple atlases are used rather than a single atlas and in these cases, the different segmentations are combined. Atlas-based segmentation is an automatic method that reduces or eliminates the involvement that is required of a technician.